A review of plutonium(IV) selective ligands

Appl. Radiat. lsot. Vol. 48, No. 2. pp. 183--200, 1997

Pergamon PII:

S0969-8043(96)00128-5

Copyright (i 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/97 $17.00 + 0.00

A Review of Plutonium(IV) Selective Ligands N. C. O ' B O Y L E ~, G. P. N I C H O L S O N 2, T. J. PIPER-', D. M. T A Y L O R L D. R. W I L L I A M S *t a n d G. W I L L I A M S : ~Department of Chemistry, University of Wales, P.O. Box 912, Cardiff CF1 3TB, UK. and -~AnalyticalChemistry Branch, AWE Plc, Aldermaston, Reading RG7 4PR, U.K. (Receired 23 April 1996)

This review compares 40 chelating ligands of differing denticity and basicity for the recovery of Pu from process waste streams. These ligands have been shown to form stable Pu(IV) complexes and to be selective for the Pu(IV) ion. Three plutonophiles identifiedas potentially cost-effectivechelators for waste treatment are the siderophore analogue DFO-HOPO, the novel actinide-chelating diphonix resin and the amide substituted N-macrocycle TCM-12aneN~. Possible means of isolating Pu on related chelating resins are explored. Copyright ,~>~1997 Elsevier Science Ltd

Introduction

superior uranophiles, which led to the ligand calix[6]arene being suggested, as a new analytical method for uranyl, and to the design and testing of a polymeric sequestering resin specific for uranyl (Hall et al., 1994: Liyanage et al., 1995).

The chemical and radio toxicities of or-particle-emitting 239pu, coupled with its long half-life of 24,110 years, dictates that the element ought to be recovered and recycled, rather than released into the environment (Taylor, 1989; Taylor, 1995; Duffield and Williams, 1986).t A number of potential recovery operations are feasible when Pu can be separated effectively from competing ions and that its solubility can be modified using a chelating ligand. The isolation of Pu is highly speciation-specific. Chemical speciation of Pu describes the physicochemical forms in which the element is present, with respect to its valence states, the concentrations and compositions of metal-ligand complexes and the amount of free aquated metal ions. This speciation is pivotal when considering the complex chemistry of Pu in aqueous systems, and especially when kinetic data are very limited. This review of Pu(IV)-specific ligands surveys a wide range of organic ligands with respect to their donor groups and the speciation of their Pu complexes, in order to identify the best plutonophilic ligands to be considered for Pu extraction from aqueous residues and process waste streams. This follows the successful approach adopted to identify

Plutonium Chemistry A knowledge of the hydrolytic behaviour of Pu is tmportant for predicting Pu mobility in waste solutions. Each of the different valence states undergoes hydrolysis; the tendency so to do decreasing in the order Pu(IV) >>Pu(VI) > Pu(III) > Pu(V). The lower valence states (III and IV) form hydrated cations while the high valencies form the dioxo cations PuO;~ and P u O ~ . Pu(IV) is the most readily hydrolysed oxidation state, with hydrolysis starting below pH 2.0. With increasing pH, the (IV) and (VI) states can undergo polymerization, whilst disproportionation is a common feature of the chemistry of the (IV) and (V) states. These two reaction types are considered to be insignificant at the typical environmental concentrations of Pu (Phillips et al., 1982), but often lead to colloid formation in neutral aqueous solutions. Precipitation also occurs as a result of hydrolysis, removing Pu from aqueous solutions above pH 2.0. The type of complex (e.g. charged or neutral) and amount of precipitate formed depends on the solubility product, the pH and Eh conditions and the concentrations of each species present. Charged hydrolytic colloids usually exist in acidic and neutral solutions, with such complexes being strongly adsorbed onto suspended particles and surfaces. Neutral, amorphous or colloidal Pu compounds

*To whom correspondence should be addressed. tFormation and radioactive decay of -"~Pu, daughter of ;~Np: ,',n + :~U.--* ~t~ [I 2~, U

,

(129 Me~,')

[~

1072 McV)

~a9 235-+mm

-~Np

239

2~;~,~

~ (515 Me'~l ~ "~m

9'~Pu2411 , Io4 ..... ~,_, U

(Excess neutrons produced by the fission of -'~sU are captured in ?~!~Uto yield -'~gPu.) ARI 48 2 B

183

184

N. C. O'Boyle et al.

include hydroxide, oxide and hydroxycarbonate species and are considered intermediates in the formation of the crystalline complexes, observed at high pH. Pu(OH)aqcl and PuO_~c, are the two most important compounds of Pu in environmental chemistry (Kim and Kanellakopulos, 1989; Rai, 1984). All the above reactions (e.g. polymerization, disproportionation and precipitation) mask the hydrolysis of tetravalent Pu, wherein a multiplicity of labile species may form simultaneously and thus preclude an accurate interpretation of the solution chemistry at equilibrium. For these reasons it is often difficult to obtain experimentally a formation constant value (13 value), which agrees closely with the theoretical value based on the free energies of formation (AG°). The highly complex solution chemistry of Pu, coupled with its highly toxic nature, greatly contributes to the scarcity of thermodynamic data, with respect to many aqueous Pu species. These facts emphasize the acute need for reliable data which can be employed to model the speciation of Pu in radioactive waste solutions.

Designing a Plutonophile In the past 50 years a major driving force for the design and synthesis of specific ligands for Pu, and related actinides, has been the need for an effective chelator which could be used to enhance the excretion of-~39pu and 241Am from the human body following significant accidental intakes. In the human body, Pu entering the blood rapidly associates with the Fe-transporting and storage proteins, transferrin and ferritin, and with the components of bone; it also forms insoluble deposits within the cells. Thus any effective chelator must be able to mobilize Pu from its binding sites in cells and tissues and complex the metal effectively at physiological pH, generally 7.4. Ligands which are to be administered to humans to remove or decorporate Pu must therefore meet certain clearly defined criteria. For example, the ligand and the resulting metal complex must be chemically and metabolically stable, at least in the physiological pH range from ~ 5 to ~ 8, the ligand must form Pu complexes that are more stable in this pH range than those formed with biochemically essential metals such as Zn, Co, Cu and Mn, and ideally the ligand should also be water-soluble and exhibit a very low toxicity. In contrast, the essential requirements for ligands to be used in the removal of Pu from waste solutions are not so rigorous. Low toxicity is not as essential and the ability to form complexes only in a narrow pH range close to neutrality is unnecessary. The speciation of Pu in the wide range of wastes likely to be encountered in the nuclear industry may be even more complicated than in vivo. Wastes may be highly acidic and contain large excesses of anions (including halides, sulphates, nitrates, phosphates, citrates and acetates), as well as a host of cations in varying

amounts that are likely to contribute to the overall speciation (e.g. Am 3÷, Ca -'+, Ga 3÷, Mg-'+, Zn -'÷ Fe-' +, Fe 3+, Ce 3+). The difficulties encountered in designing and testing plutonophiles are particularly challenging and must take account of requirements such as: (i) the isolation of Pu from complex waste streams in a form which permits its further processing into an appropriate form for re-use or safe storage; (ii) the propensity of Pu to hydrolyse and form colloidal species or precipitates over a wide range of pH; and (iii) the scarcity of experimental data due to the radiotoxicity of Pu and the associated stringent requirements for laboratory studies. For a ligand to effectively scavenge Pu ions from a complex matrix, the formation constants of the resultant Pu-ligand complex(es) need to be sufficiently large for the ligand to compete with other metal-ligand complexes and with insoluble Pu species, and thus ensure Pu solubility over a wide pH range. The electronic properties of the metal ion should match those of the donor atoms of the ligand according to the principles of hard and soft acids and bases (Pearson, 1990). Although Pu(IV) is considered to be more covalent in character than comparable Ln(III/IV) ions (where Ln denotes any element in the lanthanide series), it is generally classed as a hard acid which exhibits (a)-type behaviour in its coordination compounds. Consequently the most effective donor atoms in order of decreasing binding power are O > N > S. For different ligands with the same donor atom, the general order of increasing stability parallels increasing basicity of ligand (measured by the pKa of the conjugate Lewis acid). Thus, in the series OH (hydroxide) > CO3- > C6HsO-(phenolate) > R C O O - (carboxylate) the O donors have pK values of approximately 14, 10.3, 10 and 5, respectively. The ligand cavity size is dictated by the ring size or by the number of bridging groups in a linear chain. This cavity ought to match the size of Pu(IV) with correctly located donor groups which satisfy the preferred steric configurations of the metal ion. It follows from previous studies with uranyl that metal ion specificity is more likely to be achieved with relatively rigid macrochelating ligands which show metal-metal discrimination through geometric constraints (Hall et al., 1994). The coordination number is generally eight in An(IV) complexes (where An denotes any element in the actinide series). The role of ligands in influencing the stereochemistry of eight-coordinate compounds has been examined by calculation of ligand-ligand repulsion energies (Blight and Kepert, 1972; Kepert, 1978). A qualitative assessment of the idealized geometry of an eight-coordinated Pu complex is often described in terms of cubic distortion, forming polyhedra of lower energy and which closely resemble

A review of plutonium(IV) selective ligands a dodecahedron, square antiprism or bicapped trigonal prism. Alternatively, an intermediate structure is possible (e.g. between the dodecahedral and square antiprismatic stereochemistries) indicating easy intercoversion and a non-rigid arrangement. Thus an ideal ligand for Pu should allow eight coordination and a similar geometry to that of the coordination sphere. The concept of generating selectivity for large

OHI HOOCCH2-- ~-- CH2COOH COOH

HO(~CH2

/ CH2COOH

H203PCI"12

Oil)

H2COOH

HOOCCH2~ /H2COOH j~N(CH~)20(CH2)~N~ HOOCCH2 r ~H2COO H (VD

~CH2PO3H2 (v)

HOOCCH2~ ~H2COO~ccH2COOH _N(CH2)2N(CH2)2N. (VII)

'C. COOH

H ~ 2 N ~ "CH2CH2OH

(II)

HOOCCH2 ~ / /N(CHz)2N

(IV)

HOOCC./

/CH2CH20 H

~CH2COO H

(bis )

jCH2COOH HOOCCH2N~ --CH2COOH

metal ions, like Pu(IV), over smaller metal ions is not simply a matter of exceeding the maximum coordination of the small metal ions. Hancock and Martell (1989) have shown that larger metal ions such as Pu(IV) exhibit stereochemical preference for ligands which form five-membered rings. Such ligands provide less strain energy with metals featuring long metal to ligand bonds of about 2.5 ,~, and small L-M-L bond angles approximating to 70 (Hancock et al., 1989). Therefore a superior

/ N(CH2)2N

HOOCCH2

(I)

185

H~H2

~

~H2COOH /CH2COOH .N(CH2)2N(CH2~N(CH2)2N HOOCCH2/ ~H2COOH~CH2COOH (VIII)

o('--°

A

,o.) (IX) (bis )

O0 (bis )

O CH3- (CH2)3- OJ ~ % OO - !

(fH2)3 CI-I3 (Xl) (bis )

(CH2)3"CH3

C6H5~ uO I1° /(CHz)3CH3 e --CH2mC-- N CH3(CH2h/ %(CHz)jCH3

0

0

CH3(CH2)5%11 __ H / CH2CH3 _ _ P -- CH2=-C-- N CH3(CH2)50f NCH2CH3

(Xlll)(bis ) (XII) (bis ) Fig. 1. The chemical structures of agents which complex with Pu(IV). The species formed may be assumed to be one ligand for Pu(IV) in each complex except where indicated as bis, tris, tetra etc. The corresponding formation constant data are presented in Table I. See also Figs 2 and 3.

186

N.C. O'Boyleet al.

plutonophile design ought to restrict ring formation to five-membered rings. In addition, the decontamination of industrial waste solutions requires a ligand which is easy and

o

R

R

o

II

relatively cheap to synthesize in high yields, exhibits good solubility properties in acidic media and possesses selectivity for Pu(IV), particularly in the presence of Fe(III) and Am(III).

R

II

0

O~'

~ N/C~c/C~N / I I I R' R" R'

CH3CH~O

c/NXR ,,

II

0

(XIV) (bis) DMDBTDMA R=CH3, R'=CaH9, R"=CIaH29

(XV)

(XVI)(bis )

o

~...~,, _ _ ,Xo. HOOC3HC--N~

O~ N - -

./

CH3COOH

.3 0

x~/~N/

~o//'-

~o

%H ,,

--~oo~ (XVlll)

(XVlI)

o

OH O ~

0

NH2

CONH

Ju

CONH

~.g

OH 0

(XlX)

l~

OH

(XX)

H

o _~'~o

OH II I C--NH o

C--N~ (XXIIl) (tetra)

(XXil)

O

H3C - - H 2 C N

O

/SH

tt

/ c ~ / C-'~i-13 CH2

OCH3

(XXIV)(tetra)

H3C --H2C /

~C~

(XXV) (tetra) Fig. 2

SH (XXVI) (tetra)

CH2CH3

A review of plutonium(IV) selective ligands

Pu(IV)-ligand

Interactions

The chemical structures for a number of promising Pu-selective ligands, designed since 1946, are

x

Y~H

OH

"~=--OH

summarized in Figs 1-3 and their Pu(IV) formation constants are given in Table 1. In describing the equilibrium between a metal ion and a ligand, the

H

OH

H

H

y,

187

(CH2)~

(XXVII) X 3,4,3 LICAM(S) H 3,4,3 LICAM(C) .CO2Na

Y -SO3Na H

)

HN-- (CH2)3~ N - - (CH2)3 - - N - - (CH:z):¢"NH

~=o

¢=o

(C~[ah

¢=o

¢-o

/ (CI"I2)~

~"N_c; omc I i!

,4".~.-OH ,#'..~..o~ ,~,.~..OH,~.~..OH

Ira-

CONH / CHzh (C~2),)CH,),

(CH,):~ fl.i,

--N~C" I II OH O

OHO .sOH

~N~C I II OH O

(XXX)

(xxlx) ~H3

/%

%/

BIN I (CH2)2 ! H H N= (C1"I2)2--N-- (CH2)2--N

f N

\d/.O

~'~H o~ (XXXll)

! CH3

I CH3

ed"-Xf"Xf"~C"%H

,OH

H(~-~O

(XXXIII) (bis )

H

~

'tOH

uo,,,)

(XXXlV)(bis )

t,.,OH

(XXXV) (bis )

O

H ~ N ~ N L o H (XXXVI)

ao°~/"-ko. L,./oH

O

H~--~./--'~'y
(xXXVllibis )

(xXXVlll) Fig. 3

(XXXlX)

188

N . C . O ' B o y l e et al.

Table 1. The formation constants for some aqueous Pu(IV) complexes obtained from the literature. The chemical structures of the agents are given in Figs 1 - 3 . The overall formation reaction is expressed as follows: [MeLqHr] pM + qL + rH~MpL~H,log flpq, = [M~,[L]q[H-],

Ligand I IP VIP XVI XX ~ XXII

log Kj, log K,,...,log K, 6.38, 4.78. 3.16 10.27, 6.19, 2.79. 1.99, 1.55 10.45, 8.63, 4.28, 2.6, 2.0 10.85. 10.10, 9.07, 5.94, 3.70, 2.07, 1.3, 0.0 10.79, 9.55, 8.96, 8.32 8.79

XXIIP XXlW t XXVI

8.15 8.8 13.0, 9.22

XXI, XXVII, XXVIII XXIX, XXX

12.1, 8.4

log K~

log fl~m,,

Ionic strength (mol dm ')

--

15.34 25.7

28.5 (110) 25.7 (110)

0.5 1.0

Nebel (1966a) Krot et al. (1962)

-Fe ~*

29.5 29.74

29.5 (110) 29.7 ~ (110)

0.5 0.1

Piskunov and Rykov (1972) Turowski et al. (1986)

---

30.8 12.73

0.7 0.1

Jarvis et al. (1996) Baracus et al. (1966)

--

11.5 10.5 17.72"

30.8 (110) 12.73 (110) 41.35 (140) 34.1 (140) > 30.0 ~ (110) 3 0 - > 50° (110)

1.0 0.1 0.1

Chmutova and Zolotov (1964) Rydberg (1956) Agarwal and Mehrotra (1963); Sofen et al. (1979) Weitl and Raymond (1980)

> 30~ (110) 16.3P (113)

--

Weitl and Raymond (1980)

0.1

Gopalan et al. (1992)

31.88 ~ (110)

0.1

Gopalan et al. (1992)

17.1P (113) 24.2 (110) 30.42 (121) 24.38 (120) 20.97 (111) 21.04 (120) > 18.0e > 21.0 °

0.1 0.1 0.1 0.1

Gopalan et al. (1992) Jarvis and Hancock (1994) Jarvis and Hancock (1994) Jarvis and Hancock (1994)

0.1 0.1 0.1 0.1

Jarvis and Hancock (1994) Jarvis and Hancock (1994) Jarvis (1994) Jarvis (1994)

M" + analogue

Th 4~ , Ce 4+

5.78, - 0.9

--

XXXI1 (meta) XXXII (para)

10.22, 9.6, 9.05, 8.31, 6.4, 5.35 10.18, 9.64, 9.0, 8.17, 7.0, 5.36

Th 4+

XXXIII XXXIV XXXV

8.11, 6.88, 1.87 8.732, 7.00. 1.7 10.06, 1.55

XXVP XXXVIP XXXVIII XXXIX

8.80, 8.24, 2.69, 1.20 8.14, 1.7

(1)

Th 4+

31.88"

15.61 ----

17.39 13.89

Reference

*Protonation data are not measured by the same author. Reliable values are all reported at an ionic strength of 0.1 mol dm-3.

stepwise (log K) and/or overall (log 13) formation constant is reported in terms of the reaction with the fully deprotonated ligand (L in equation (1)), where charges have been omitted for simplicity. When such a formation constant has not been determined, the appropriate analogue was used (e.g. Th 4+, Fe 3+ ) to give an extrapolated value denoted by log 13a. Alternatively, an estimated value, based on comparable Pu-ligand interactions, was reported as log 13~. A notable consideration from a thermodynamic viewpoint is that not only are the magnitude of the log 13 values important in assessing the relative affinities of a particular ligand for a metal ion, but also the size of the pK~ values and therefore the degree of ligand competition for protons, is an essential requirement. Authors who have pioneered the search for a Pu-specific ligand used sequestering agents known to be effective at binding elements with similar properties to Pu (e.g. ions of Ln, U, Th, Fe and radio-Sr as described by Painter et al., 1946; Schubert and Wallace, 1950; Foreman and Hamilton, 1951; Foreman and Finnegan, 1957; Foreman, 1960; Kawin and Copp, 1953; Kroll, 1956; Vaughan et al., 1973). This heuristic approach focussed on ligands containing carboxylate or hydroxy functional groups (e.g. citrate 3-, N,N-bis(2-hydroxyethyl)-glycinate, nitriloacetate, ethylenedi-

aminetetraacetate 4- and substituted analogues, etc. (structures I-VIII, Fig. 1). However, relatively few of these polycarboxylates are able to discriminate against other metal ions in favour of Pu(IV). Successes in administering these ligands as chelation therapy were limited for a number of reasons, given by Raymond and Smith (1981). Although the administration of diethylenetriaminetetraacetate 5- (DTPA 5 ) significantly reduces the body burden of Pu, there are problems with solubility and toxicity. These matters are discussed later in their appropriate category. Five categories of potential plutonophile are reviewed in this section: 1. 2. 3. 4. 5.

ligands for solvent extraction; ion exchange plutonophiles; polycarboxylates; synthetic derivatives of the siderophores; substituted N donors.

A range of multidentate and macrochelating agents were surveyed and a shortlist prepared, of those exhibiting superior plutonophilic behaviour. The flow diagram (Fig. 4) shows the schematic process of elimination through which ligands, thought to be highly selective for Pu(IV), were identified.

A review of plutonium(IV) selective ligands PLUTONIUM(IV)

/

I

/. ~ l

derlviltlveoSynthotVJCof

- SELECTIVE

I

[

.ydroxamates,]~ Catecholamides,

~'l

/ SPECIFIC

LIGANDS

3,4,3 LI-HOP~

' -DFO - - -HOPO I " (b)

~

[HydrOxypyrldOmdes]

slaer°l~°res

(-lT---X " / ~-N~N~" C~._(de

sferrioxamineB) O

I

Ligandsforsolvent I l extract on I

/

.

= ~,]

i =.,.,,.+m r-i r ~ /

Cleating rHIrm [type(ll)]

OholWionrosins [lyll~lt(il,),

°,°...,.

F I

I '

[

+

"1

Octadentato /

| "

Pt~lur~d'w~vvl~tA~ [ ................

I

Subitilutlld ~ nltrogendonors ~ ' ~

.~Hh~fili l i V n n l a m l n o ~

189

=

.o,#.o.

I-fI I ~?

CH ~ "

CH/'~

I~ '; ~

~

,

\

/C C H / ~ " CH "~'~ C ,

~ ,,¢,, . _,-~.,, ~ ~ I t~L~ ~1~1 (~, ~lill~'U 0 OH % y ) o 3n / \ SO+H x ] .. i N (A representative segment of a resin s~and) / n

/HP'12anoN4

~

OH

/

]

I (Hydr°xyalkyl gr°ups_)l

l ~ I(c~ O ~ I ~ i TCM.12aneN 4 I - • ~NH2 /S ~ "] Sul0eUluted cyclens~.---'-"~ - I (Arnidogroups) I L

j O N\

J

Nl-b

I

I

Shottlist of ligands identified in the fiteratttre as having a high affinity for Pu(W).

O~

"+"--/ "--'~0 -

Fig. 4

Ligands for Solvent Extraction Extraction of Pu from an aqueous system which usually contains nitrate, into an organic phase (non-polar or slightly polar), utilizes to good advantage the differences in extractabilities of the various oxidation states of Pu and other actinide elements. Experience has shown that the most readily extractable states of Pu are the tetra- and hexa-valent, with the trivalent and pentavalent states being poorly extracted. In the case of Pu(IV) and Pu(VI), neutral complexes, Pu(NO3)4 and PuO2(NO3):, are formed prior to solvation by two organic solvent molecules. Extraction o f neutral complexes Crown ethers. Liquid-liquid extraction of Pu(IV) by crown ethers from various aqueous-organic solutions has been extensively researched by Shukla and Lohithakshan (1989) Shukla et al. (1991, 1993) who reported the abilities of a series of crown ether compounds to scavenge Pu(IV) and (VI) from nitric acid media. It was shown that the size of the macro ring and the nature of the substituent(s) in the crown ether skeleton strongly influenced the extractability for actinide ions. The sequence of the Pu(IV) extracting power of these macrocycles into toluene followed the order dicyclohexano- 18-crown-6 > monocyclo-hexano- 15crown-5 > dicyclohexano-24-crown-8 > dibenzo-18crown-6 > di-tert butylbenzo 21-crown > 1,10dithia-18-crown-6 > mono-benzo-15-crown-5 (Shukla et al., 1991). Recovery of macrocycles from Pu is easily accomplished using oxalic, sulphuric or perchloric acids as stripping ligands. Dicyclohexano-18-crown-6 (DC18C6, structure IX, Fig. 1) specifically increased the extractability of both Pu(IV) and U(VI) in acetonitrile, toluene and

benzonitrile. The apparent extraction constant into benzonitrile by DC18C6 and also DC24C8 (structure X, Fig. 1) was about five orders of magnitude larger for Pu(IV) than for UO~ + with greater than 95% extraction (Shukla and Lohithakshan, 1989). Pu(IV) was extracted as a cationic sandwich associated with nitrate ions to give a 1:2 complex, presumably of the type Pu(DCI8C6)4+.4NO{, having an overall formation constant log 13L2of 5.34. According to Rogers et al. (1991), the limited extracting power of crown ethers for large actinide ions is due to the formation of crown ether-hydronium ion complexes in acidic media, rather than the existence of an 'inclusive' Pu(IV)-crown ether complex (where the actinide ion is encapsulated by the crown ether). Rogers and co-workers have isolated crown ether complexes of H30 +, HsOf and H 9 0 4 + and suggested that the dominant feature of actinide ion complexation was likely to be formation of a crown ether-hydronium ion-actinide-ion sandwich, thus facilitating cation transfer to a hydrophobic phase. Organophosphorus ligands. Bidentate organophosphorus reagents are the most predominant agents used in modern nuclear waste decontamination and reprocessing processes. For example, bidentate tri-n-butyl phosphate (TBP, structure XI, Fig. 1) is used in the PUREX process to extract, separate and recover Pu from U and fission products. Extraction with TBP takes place from nitric acid solutions, an advantage over other organic solvents which require additional salting agents to increase nitric acid solubility in the organic phase. The reaction of Pu(IV) nitrate with TBP is well established and proceeds as follows: pu4+iaql + 4NOCi~q).

+ 2TPBI~qI~Pu(NO04.2TBP, o,+I

190

N. C. O'Boyle et al.

The versatility, resistance to HNO3, efficiency and specificity of tri-n-butyl phosphate is the reason why it remains so widely used in nuclear institutions (for large- and analytical-scale use), some 40 years after its introduction as an actinide extractant. However, actinide extraction is limited due to competition between metal ion(s) and nitric acid for TBP. Formation of the solvent extractable species HNO3.TBP at low pH values decreases the free ligand concentration (TBP). The presence of anionic nitrate complexes in the aqueous phase also hinders extraction at high acid concentrations. Consequently, alternative organophosphorus ligands have been investigated to succeed TBP, and also meet the specific, often diverse, requirements imposed on actinide extractants for complex waste systems. Ligands with a higher extracting power are required in response to the ever-increasing demand for a more efficient and economic process. In order to optimize actinide separation and recovery for better reprocessing, the ideal extractant must also: (i) extract An(Ill), (IV) and (VI) nitrates with minimal extraction of Ln(III) nitrates; (ii) be compatible with concentrated nitric acid solutions as well as the chosen industrial diluent; and (iii) have a low solubility in the aqueous phase. Octyl(phenyl)-N,N-diisobutylcarbamoyl methyl phosphine oxide (CMPO, structure XII, Fig. 1) is regarded as a highly efficient ligand for actinide extraction (Pu extraction > 99.99%), and so is used in many nuclear laboratories today. However, such efficiency limits the Pu(IV)-specificity of CMPO which, unlike TBP, extracts both lanthanide and transition metal ions in addition to actinide ions. Also, hysteresis effects are large with CMPO, causing major problems with back extraction of Pu once the stable complex Pu(NO3)4(CMPO)2 is formed in the organic phase. Mathur et al. (1992) reported the extraction behaviour of Pu(IV) from nitric acid solutions with a mixture of CMPO and TBP in dodecane and concluded that Am(Ill), U(VI) and Pu(IV) could be efficiently extracted from high-level waste solutions of PUREX origin, with actinide ion extractability increasing in the order Pu(IV) > U(IV) > Am(Ill). A resin which is a mixture of TBP and CMPO is now commercially available from Eichrom Industries, Inc. for the extraction of actinides and lanthanides (Internal report from Eichrom Industries Inc., 1995). A superior actinide extractant compared with CMPO, in terms of hysteresis, is dihexyl-N,N-diethylcarbamoylmethylene phosphonate (DHDECMP, structure XIII, Fig. 1), and was recommended for industrial purposes about 15 years ago because of its *Radioactive decay of 2]~Pu: 94Pu --~ 241 241 9sAm ( > 99%) 144 years (49 MeV) 2~Ptl ~ 237U92 ( 1 0 - 3 % ) 144 years

high extracting power of > 99.99% 241pu and 241Am extraction (Marsh and Simi, 1981). The fact that D H D E C M P is not Pu(IV)-specific could prove advantageous in the extraction and separation of Pu and Am. Am is produced in all Pu waste solutions by the radioactive 13 decay of-'4~Pu.* The degree of a-particle emission from Am is extremely small in comparison with Pu. It follows that the older the Pu waste, the more Am will be present and the greater the radioactivity. If only Pu is extracted (as with TBP in nitrate systems), the difficulties associated with Am extraction from trivalent lanthanides remain. For these reasons recovery operations usually involve five or six separation steps. An ideal means of separating lanthanide(III) ions from actinide ions would be to reduce the multiple stage process to a single chelation step. Here the selective properties of soft donor ligands could be used to good advantage. The soft N or S donor atoms form metal-ligand atom bonds with greater covalent character than O. Consequently, soft donor ligands have a higher affinity for actinides than for trivalent lanthanides (Musikas, 1984). For example, Eichrom Inc. have developed an effective method for Am(III)-Ln(IlI) separation based upon their slight differences in covalent character with respect to thiocyanate. Diamides and picolinamides. Diamides and picolinamides were evaluated by Nigond et al. (1995) for the treatment of nuclear wastes; these extractants have the advantage of being totally combustible. Results showed that malonamides could extract actinides (Ill), (IV) and (VI) from nitric acid solutions. N,N'-dimethyl N,N'-dibutyl tetradecyl malonamide (DMDBTDMA, structure XIV, Fig. 2) formed neutral complexes with Am(Ill) nitrate and Pu(IV) nitrate in tetrahydrogenated propene (TPH). New malonamides were studied by modifying the R, R' and R" aliphatic groups within (DMDBTDMA). Results confirmed that the two substituents on the N atoms must be different or assymetric in order to optimize solubility in aliphatic diluents. In the presence of metallic salts, a sufficient number of C atoms on the N and the C bridge substituents were also necessary to avoid third-phase formation (or non-miscibility in organic media). Hence the diamides (CH3(C4H9)NCO),CHR" with R " = C18H37 or C16H33 appeared to be the most suitable molecules for further investigations. Picolinamides combine the advantages of amides which feature O donor atoms with the selective properties of soft N donor atoms (Musikas, 1984), as in structure XV, Fig. 2. Thus these extractants have the potential for separating An ions from Ln(III) ions. As with the malonamides, the chemical properties of picolinamides depend largely on the nature of the R, R' and R" alkyl substituents. However, the main drawback with the use of these molecules was their solubility in common diluents. The relatively high An(IV) extracting power of

A review of plutonium(IV) selective ligands pyridine 2-carboamide ( R ' = R " = H), determined by Raman spectroscopy, indicated that An(IV) ions could be readily separated from Ln(III)ions (Livet et al., 1993). A shift in the carbonyl and pyridinic ring stretching vibrations also implied bidentate complex formation through the carbonyl and the N of the pyridinic ring. Extraction by anions

The monobasic extracting agent di-(2-ethylhexyl) orthophosphoric acid (HDEHP, structure XVI, Fig. 2) is the most important anionic organophosphate ligand, particularly for the reclamation of Pu(IV) in transuranic element production, commonly known as the PUBEX (Pu-Bk extraction) process. Pu(IV) is completely extracted from trivalent transplutonium elements by HDEHP in diethylbenzene. Recent studies have shown that HDEHP acts as a 'liquid ion exchanger' removing Pu 4+ and PuO~ + from an aqueous solution of pH 0, and also Pu 3+, PuO + and if dichromate is added, thus leaving only polymeric Pu(IV) in the aqueous phase (Neu et al., 1994). Solvent extraction of Pu(IV) from aqueous solutions containing ionisable macrocyclic ligands has been investigated by Manchanda and Mohapatra, 1990. The 15-member ring ligand, 1,7-diaza4,10,13 trioxacylopentadecane N,N'-diacetic acid (K21DA, structure XVII, Fig. 2) forms a strong 1:1 complex with Pu(IV) of the type Pu(K21DA) 2~ in TTA, the overall log formation constant, log [3~L is 21.52. The high stability of the K21DA complex is attributed to the "'encapsulation of the metal into the ligand cavity" and hence a greater metal-ligand interaction. The major disadvantage in using liquid liquid extraction techniques for the recovery of Pu from nuclear processes is the accumulation and disposal of large quantities of organic solvents as radioactive effluents. Increased restrictions on the discharge of these low and medium level radioactive waste products reinstates the application of ion-exchange treatments as the preferred approach to radwaste entrapment.

Ion Exchange Plutonophiles Ion exchange resins fall into three broad categories: (i) general cation and anion exchangers; (ii) chelating resins where several chelating groups collectively complex to metal ions; and (iii) chelation resins where just one ligand, such as calix[6]arene, complexes a specific metal ion (such as UO~ +). As a versatile, inexpensive and widely available means of separation, general cation exchangers [type (i)] proved an acceptable method of extracting a range of actinides from different waste matrices (extraction > 95% assuming optimum conditions). However, this treatment was a completely nonspecific means of isolating Pu in highly acidic waste

191

solutions, containing an excess of alkali and alkaline-earth cations. Also, a second recovery operation is often recommended and further effort is required to regenerate the resin, especially if its exchange capacity is affected by components in the waste being adsorbed onto the resin. To overcome problems with selectivity, a number of chelating ion-exchangers were prepared, some being commercially successful (Korkish, 1989; Calmon and Simon, 1979). However. chelating ionexchange groups are generally weak acids with little or no ionic dissociation and so they exhibit low hydrophilicity in highly acidic media. Another major concern was the poor kinetics of the metal uptake; the exchange process in a chelating resin is often slower than in strong-acid resins of the sulphonic type (Korkish, 1989). Those chelating ion-exchange resins [type (ii)] containing phosphoric [-OPO(OH)2], phosphonic [-PO(OH)2] or phosphinic [-PO(H)(OH)] acid groups, known as medium acidic cation-exchangers, did not offer any significant improvement over common cation-exchangers [type (i)] (Calmon and Simon, 1979). However, Alexandratos et al. (1987) recognized that resins which incorporated phosphorous acid ligands were able to coordinate metal ions through the phosphoryl O atoms, in solutions of pH appreciably lower than the ligand pK~ (pK, = 3-4). For these reasons, they were considered better than sulphonic-type ion-exchangers in acidic media. Katz et al. (1986) have discussed the use of many anionic exchange resins for capturing actinides, but their specificity is generally poor when compared with cationic exchange resins, and for this reason such resins have been excluded from this discussion. Diphosphonic ion exchange

Actinide ion complexing was further improved by substituting the more acidic methane diphosphonic acid groups (pK, < 1.5) in place of monophosphonic acids (Rizkalla, 1983). Recently, the properties of a new polythnctional cation-resin, containing geminally substituted diphosphonic acid groups chemically bonded to a sulphonated styrenic based polymer matrix, were described in a series of six papers by Horwitz et al. ( 1993): Horwitz et al. ( 1994): Chiarizia et al. (1993): Chiarizia et al. (1994): Chiarizia and Horwitz (1995h Nash et al. (1994). This resin, called diphonixTM ( d i p h o s p h o n i c ion exchange) demonstrated a remarkably strong affinity for actinide ions in acidic media (including Am(lII), Th(IV), Np(IV), Pu(IV) and U(V1)). Also, diphonix proved more effective when compared with the commercial sulphonic-type strong acid cation-exchanger BioRadTM AG HP-50 and a sulphonated monophosphonic resin. Such improved extraction was attributed to the relatively high acidity of the actinide-specific, gem-diphosphonic acid group (Nash et al., 1994), and its capacity to chelate actinide ions through either

192

N. C. O'Boyle et al.

ionized or neutral diphosphonic acid ligands, leading to metal complexes of high stability. Additionally, the presence of hydrophilic sulphonic groups facilitated the access of ions into the polymer, and so increased the rate of actinide uptake by diphonix. The most likely conformations of the actinide ion-diphosphonate complex were reported by Horwitz et al. (1993). Minimum energy conformation analysis showed that the phosphoryl O atoms and acidic O atoms cannot simultaneously coordinate to the metal ion, thus diphosphonate is considered as bidentate (Horwitz et al., 1993). Researchers concluded from distribution studies that, in most cases, the uptake of Am(III) and U(VI) was explained by assuming that the uncomplexed metal ion was sorbed (similar to conventional sulphonic type resins). Conversely, tetravalent actinide uptake by diphonix was achieved via chelating of neutral actinide(IV)-nitrate complexes by two phosphoryl groups as shown in Fig. 5, and is analogous to solvent extraction by organophosphorus compounds. (Some evidence of uptake of neutral uranyl complexes was found, from pH neutral solutions.) Furthermore, all investigated actinides could be efficiently and rapidly stripped from the diphonix resin with a 1 M solution of the chelating agent HEDPA (1-hydroxyethane-l,l-diphosphonic acid). Recovery rates with HEDPA were of at least 90% in about 2 min contact time. Thus it was concluded that the use of diphonix and its salts could be applied to any process, where removal of metal ions from acidic or neutral waste effluents was required (Horwitz et al., 1994). (This resin is now commercially available from Eichrom Industries, Inc., Darien, Ill.) The efficiency and reversibility of diphonix on an industrial scale is yet to be reported in the literature. In practice, such high stability of Pu complexes could cause problems with back extraction.

Polycarboxylates Many ligands containing carboxyl functional groups, especially the polyaminopolycarboxylate

compounds such as ethylenediaminetetraacetate (EDTA) or diethylenetriaminepentaacetate (DTPA) may be considered to be potential plutonophiles. Today, DTPA, as its Na3, Ca or Zn salt, is the current agent of choice for the decorporation of Pu or Am following accidental intake in humans. However, it is by no means 100% effective and its potential toxicity limits the achievement of optimal chelator concentrations at the sites of Pu deposition. While the potential toxicity of DTPA is not an important consideration when applied to industrial waste treatment, its biodegradability, which is as yet poorly understood, could be a cause for future environmental concern. However, there are other drawbacks to DTPA and the polyaminopolycarboxylic acids as plutonophiles. Not all the eight donor groups on the DTPA molecule may be sufficiently compatible with the preferred steric configuration of the Pu 4+ ion to allow effective recovery of Pu(IV) and other ions such as citrate 3and Fe(II/III) are potential interfering agents. Indeed, the high DTPA concentrations needed to complex Pu(IV) in the presence of citrate will thermodynamically favour Fe(III) binding. Two relatively new dihydroxamate analogues of the siderophore aerobactin, derived from EDTA and DTPA (di-isopropyl-EDTA-DX and DTPA-DX, respectively, structure XVIII, Fig. 2) have been studied by Turowski et al. (1988). These ligands were easy to synthesize in large quantities with good solubility properties. Thermodynamic evidence suggested that both ligands had a higher affinity for Fe(III) than their respective carboxylate derivatives and were therefore regarded as promising candidates for Pu chelation therapy. Although i-Pr.,DTPA-DX has more carboxylate and amino groups available for complexing tetravalent metal ions, this ligand proved no more effective than DTPA in animal tests with injected 238Pu and 24~Am. Octadentate bisamide derivatives of DTPA were also investigated (Roth and Raymond, 1995). For the bis(ethylamide) and the bis(methylamide) derivative of DTPA (structure XIX, Fig. 2), the magnitude of

\/ C

HOx/\/OH .o/\

O

\/ (a)

I \o.

O

.ol\X o o//"-o. % III

t

Co)

Fig. 5. Probable conformations of complex formed between actinide ions and the diphosphonate ligand.

A review of plutonium(IV) selective ligands the contribution of the amide functional groups to the stability of Gd complexes was researched. Comparative potentiometric studies with the parent ligand, DTPA, by Roth and Raymond (1994) showed that the amide functional groups contribute significant stability to Gd complexation whilst increasing the relative selectivity for Gd ~+ over Ca-' +. Replacing carboxylate groups with coordinating amide O atoms in this manner would also improve a ligand's selectivity for Pu.

Synthetic Derivatives of the Siderophores

log KFI

Group

(a)

II

R-- C-- N--H Hydroxamic Acid

9.36 (R = CH3)

(b)

II

R-- C-- N--R' Thiohydroxamic Acid

(c)

Introduction A biomimetic and stereonostic approach to the design and synthesis of a ligand series was adopted for the specific role of complexing Pu and other actinide ions in humans (Raymond and Smith, 1981). These synthetic molecules were based on the chemical and biological similarities of Pu(IV) and Fe(III) and on the knowledge that microbes produce highly specific Fe(III) sequestering agents called siderophores. Similarities between Fe(III) and Pu(IV) range from their charge to ionic-radius ratios (4.6 and 4.2 e ,~ -~, respectively) and formation of highly insoluble hydroxides (K~o ~ 10 39 and 10 -55, respectively) to their broadly similar transport properties in mammals. The structures of two siderophores are shown in Fig. 2 (structures XX and XXI): the hydroxamate and catecholate groups are the most common ligating groups, but similar O-containing donor groups (e.g. hydroxy acids) are also found.

193

~

<9.0

OH OH

13.0, 9.22

Catechol (d) N - (CH3)2

I

----~ OH OH

(e)

12.1, 8.4

N,N-dimethyl-2,3-dihydroxylbenzamide

~

N ~

OH OH 1-Hydroxy-2(1H)-pyridinone ( 1,2-HOPO) (0

5.78, -0.9

H

8.65, 0.11 3-Hydmxy-2(1H)-pyridinone

(3,2-HOPO)

Actinide binding and solubilization by siderophores Interactions between actinides and the siderophores were considered to be environmentally important because these chelates bind hard Lewis acids with extraordinary affinity (as shown in Table 1) and thus solubilized large metal ions in nature from insoluble phases (Bossier et al., 1988). Therefore these ligands were expected to form highly stable complexes with Pu. Quantitative studies by Brainard et al. (1992) suggested that siderophores also had the potential to mobilize actinides in the environment. The measured rate constants for solubilization of insoluble actinide oxides showed enterobactin to be about three orders of magnitude more effective than several strong chelators (notably polycarboxylates) at increasing the rate of solubilization of hydrous Pu oxide. This confirmed that functional groups found in siderophores (Fig. 6) would also be good as Pu complexing agents. Hydroxamate ligands From a practical viewpoint, the versatility of small bidentate hydroxamate anions may offer certain

Fig. 6. Early Pu sequestering agents. advantages over large polydentate synthetic macrochelates, where commercial success is often hindered by limited availability, low efficiency, expensive synthesis and low water-solubility. Also, the steric constraints of the larger molecule and incompatability of donor sites located around the central atom may reduce its specificity for Pu. Explanations regarding the possible structures of the hydroxamate anion often remain rather tentative in pertinent literature, although considerable p r o gress was achieved with the evolution of sPectral methods (Mathis, 1953; Mathis et al., 1961; Exner, 1968; Coutts, 1967a: Coutts, 1967b; Smith, 1966: Sandier and Karo, 1972). Thermodynamic investigations by Monzyk and Crumbliss (1979); Monzyk and Crumbliss (1980) rationalized variation~s in AH°,AS° and pK, with changes in RL and R, substituents. This led to a clearer understanding of hydroxamic acid chemistry with respect to proton dissociation, influence of R substituents on acid strength, stability of the conjugate base anions and

194

N. C. O'Boyle et al.

~ /

1 OH

C--N

-"" C - - N

\

RI

/

\

R2 R1

I

~

C~N

/

R2 R1

II

+ \ III

~ R2

-~

CmN

when R 2

+

~

--C6H5 /

R1 IV

Fig. 7. Resonance forms of hydroxamic acids (positive and negative charges represent formal charges on the hydroxamate ion). metal complexation. Equilibrium studies by these authors have shown that the stability of hydroxamate complexes can be interpreted on the basis of resonance forms II and III (Fig. 7) (Monzyk and Crumbliss, 1979; Monzyk and Crumbliss, 1980). It is generally agreed that for N-substituted hydroxamic acids (R_, :~ H, structure XXIII, Fig. 2), proton dissociation occurs at the OH site, due to the electron density donating ability of R2, which helps stabilize resonance form III, relative to II by induction. However, for non-N-substituted hydroxamic acids (structure XXII, Fig. 2), it is argued that the solvent system plays an important role in proton dissociation and thus the -NH proton may be ionized. When R2 C6H5, resonance delocalization of the lone pair on N (via resonance form IV) enhances the stability of the anion (in addition to inductive effects) by providing a build-up of positive charge on N. Thus the weak acid N-phenyl-benzohydroxamic acid (NPBH, structure XXIII, Fig. 2) ionizes to produce hard O donors which complement the high Lewis acidity of the hard Pu(IV) ion. This is supported by large formation constant for Pu(IV)-N-phenylbenzohydroxamate interactions (log I~l~ = 11.5, log 1~4 = 41.35), log 13, being three log units higher than the corresponding U(IV) and Th(IV) complexes (Chmutova and Zolotov, 1964). Modelling experiments performed in this laboratory predicted that NPB will compete favourably for Pu(IV), allowing 100% complexation under optimum conditions and above pH = 6.0 in the presence of strong Pu chelating agents, such as citrate 3-, acetate and halides (O'Boyle and Williams, 1995). Furthermore, the complexing power of NPB- for Pu(IV) (atomic radius = 0.93 A.) under conditions specified by Chmutova and Zolotov (1964) is much stronger than for Fe(III) (atomic radius = 0.67 A.) since Pu(IV) forms the highly stable complex Pu(NPB)4. =

O c t a d e n t a t e complexing agents

The synthesis and analysis of tetrakis-ligands allowing eight or nine coordination to a central lanthanide or actinide received considerable attention in the 1970s with respect to their structural arrangements and An/Ln analogous relationships (structures XXIV and XXV, Fig. 2). Following

preliminary studies with Hf(IV) and Zr(IV), Th(IV) tetrakis-(N,N-diethyldithiocarbamate) [Th(Ebdtch] emerged as the first complex confirmed by structural studies to have eight sulphur atoms coordinated to a central atom (structure XXV, Fig. 2, Brown et al., 1970). The solid state structure was determined as an intermediate between dodecahedral and square antiprismatic. Re-examination of the shape parameters of eight coordinate metal(IV) tetrakis complexes (e.g. structure XXIV and XXV, Fig. 2) by Steffen and Fay (1978) demonstrated the ease with which the coordination geometry may be mis-described if the assignment is based upon a single set of parameters. For example, further calculations of bond angles and determination of 8 and qb shape parameters for Th(Et2dtc)4 provided evidence that the stereochemistry was much closer to the dodecahedron (stereoisomer) than to the square antiprism. Thus it was concluded that such close relationships between idealized eight-coordinated geometries justified the provision of tabulated 8 and q~ shape parameters, in addition to appropriate mean-planes calculations, in defence of an assigned geometry. Advice by Steffen and Fay (1978) was acknowledged in the stereonostic approach adopted by Sofen et al. (1978); Sofen et al. (1979) who developed the first synthetic analogues of siderophores based upon the biomimetic concept (structure XXVI, Fig. 2). The isostructural tetrakis-(catecholato) complexes of Ce(IV), Th(IV) and U(IV) were reported as having idealized trigonal-faced dodecahedron geometry (D.,d molecular symmetry). Examination of molecular models showed that ligand specificity for actinide(IV) ions was further optimized if four catecholate groups were attached to the N atoms of a series of cyclic tetraamines via amide linkages, as shown in structure XXVIII, Fig. 3 (Weitl et al., 1978; Weitl and Raymond, 1980). It was also shown that conjugates of cyclam-14 (n = 2) and cyclam-16 (n = 3) would optimize ring size and increase selectivity for Pu(IV). Catecholamides. Linear (Li) and cyclic (Cy) catecholamides were reviewed by Raymond and Smith (1981) in terms of their potential as Pu-specific chelators for decorporation therapy in vivo. The most promising of these were the linear sulphonate, or carboxylate derivatives 3,4,3-LICAM(S) and 3,4,3LICAM(C) (structure XXVII, Fig. 3). Animal studies

A review of plutonium(IV) selective ligands demonstrated greater efficacy of both LICAMs for Pu decorporation as compared to DTPA (Lloyd et al., 1984; Volf et al., 1986). However, DTPA was found to be a very much more effective chelating agent for Am(Ill) than LICAM(C). This was expected since the affinity of the catechol ligands for the larger and less acidic lanthanide(III) and actinide(IlI) ions is quite low. For waste treatment, or for environmental remediation, a major drawback of the linear catecholamines is their strong association with protons under near-neutral and acidic conditions, thus restricting complexation to high pH values. Furthermore, catecholate is the most potent Fe(llI) chelator among the siderophoric chelating groups. Linear h y d r o x y p y r i d o n a t e s . As a direct development from the CAMS series, incorporation of the more acidic analogue of catechol [namely monoprotic hydroxypyridonate (HOPO)] produced a series of ligands, of which the most promising plutonophiles were: (i) the structural analogue of 3,4,3-LICAM; 3,4,3-Li(I,2-HOPO) composed of the hydroxypyridinone isomer, 1,2-HOPO; and (ii) the addition of one HOPO to desferrioxamine, DFO-HOPO (structures XXIX and XXX, Fig. 3). Biological data clearly indicated that the aqueous solubility and relatively high acidity of these octadentate ligands favoured highly stable eight-coordinate Pu(IV) complexes (Poncy et al., 1993; Paquet et al., 1994; Gray et al., 1994; Stradling et al., 1991, 1992, 1993, 1995a; White et al., 1988; Fritsch et al., 1994). These comparative studies researched the relative efficiencies of the octadentate HOPO ligands, 3,4,3-LICAM(s) and DTPA. A significant increase in Pu(IV) removal was observed with DFO-HOPO and 3,4,3-Li(1,2-HOPO). The effectiveness of early single chelate injections on ~3SPu retention in tissues decreased in the order 3,4,3-LIHOPO > DFO-HOPO > DTPA > DTPA-DX (Volf et al., 1993). Injected 3,4,3-LIHOPO proved to be appreciably more effective than DTPA for Pu removal, after administration by either intravenous injection, Pu-TBP inhalation or by simulated wound contamination. Its potency for & vivo chelation of Pu(IV) and also for Am(III) was evident in all tests on rats and mice (e.g. 88% 23~Pu and 64% 24~Am were removed in rats, Stradling et al., 1992, 1993). However, its acute toxicity at high dosage, coupled with expensive starting materials and small yields (White et al., 1988) rendered 3,4,3-LIHOPO an impractical option. Conversely, the desferroxamine derivative produced a ligand of comparable efficiency, higher Pu specificity (removing only traces of inhaled or injected 24~Am) and lower toxicity (Stradling et al., 1992, 1993; Volf et al., 1993). If the thermodynamic stability of the Pu-DFO-HOPO complex was measured, an increase in this formation constant value of Pu(IV)-desferroxamine (log [3 = 30.8) would be expected due to an increase in the number of

195

binding sites, whilst maintaining the compatability of donor atoms in a non-rigid arrangement. The apparent success of 3,4,3-LIHOPO stimulated the design of low toxicity ligands which were easily prepared (Stradling, 1994, 1995b). For example, hexadentate [ N , N ' , N " - t r i s ( l l , 2 - d i d e h y d r o 3- hyd roxy- 1-met hyl-2-oxopyriol-4-yl-(carbonyl)2,2',2"-triaminotriethyleneamine] or TREN- (Me-3,2HOPO), (structure XXXI, Fig. 3) composed of the less acidic hydroxypyridine isomer 3,2-HOPO was almost as effective as 3,4,3-LIHOPO after intravenous injections of -'SPu(IV) and -'4tAm(IIl) citrate in mice, removing 82% -'38pu(IV) and 89% ~ A m ( I I l ) (Durbin et al., 1994). Its low toxicity and easy synthesis suggested a favourable and practical compromise between the effectiveness of 3,4,3-Li,(I,2HOPO) and the safety of CaNa~-DTPA. This documented evidence regarding the use of siderophores for industrial waste decontamination suggests that the octadentate DFO-HOPO ligand could provide a most efficient, Pu(IV)-specific extractanc The synthesis of novel, cost-effective chelators for the removal of Pu and other actinides from process waste streams has been investigated by Gopalan et al. (1992) and Koshti et al. (1994). A new class of tetrahydroxamate chelator (structure XXXII, 1,4 conformation, Fig. 3), demonstrated high affinities for Th(IV), Fe(llI) and Nd(lll) when analysed potentiometrically: Th(IV) and Nd(lll) were selected as analogues for Pu(IV) and Am(Ill), respectively (Gopalan et al., 1992). Almost identical protonation constants for the 1,3 and 1,4 conformation chelates (Table 1) indicated that the position of attachment of ligand groups about the benzene ring did not significantly affect their relative basicity. However, experience with the previously discussed HOPO and CAM ligands would suggest that such weak acids might not be as effective as DTPA, particularly in the presence of competing ions in acidic media. Even at higher pH (pH > 5.0), strong competition with protonated species would reduce the ligand's ability to maintain Pu below the solubility limits of Pu(OH)4,~ and PuO2,,~. Thus, in order to maximize a chelator's potential for Pu binding it is important to minimize the limitations caused by low solubility or stability~ as well as unfavourable kinetics or selectivity. Hence molecular modelling studies were performed by Koshti et al. (1994) in an attempt to understand the structural chemistry of metal-ligand complexes in greater detail. The design of Pu(IV) chelators was based on maximization of ligand interactions with the metal ion and modification of alkyl chain lengths. This led to the identification of energetically and sterically favourable Pu(IV) complexes and the subsequent synthesis of two novel cyclam based chelators for Pu(IV). The synthesis of N,N',N",N'"tetrakis(acetohydroxamato)-, 1,4,8,11-

196

N. C. O'Boyle et al.

tetraazacyclotetradecane(CYTROX)andN,N',N",N'"-

tetrakis(acetylacetonato)-,1,4,8,11-tetraazacyclotetradecane (CYTAC) has been achieved using short sequences from the readily available cyclam. Evaluation of the binding properties of these ligands with Pu(IV) will establish the validity of the molecular models.

Substituted N Donors Jarvis and Hancock (1994) recognized that the protonation constants of polyamines, such as EN, DIEN or TRIEN, can be lowered by the attachment of hydroxyalkyl groups, as in structures XXXIIIXXXVIII, Fig. 3. Addition of such groups, which carry neutral O donors, increased the selectivity of the N donor ligand for large metal ions. In addition, complexation of metal ions, which usually have a low affinity for N, was possible. Experimental data confirmed that those saturated N donors were efficient at complexing Pu(IV) and at stabilizing the solution equilibria against hydrolysis. For example, both experimental and modelling studies gave strong supporting evidence that PHPDIEN [(N,N,N',N',N"-pentakis(2-hydroxypropyl)1,4,7 triazaheptane, structure XXXIV, Fig. 3], bearing five neutral hydroxypropyl groups, has a high affinity for Pu(IV) in alkaline media (Jarvis and Hancock, 1994). Also, log/(ME values were compared for THECHDA, THEC and BICIN (structures XXXV, XXXVI and XXXVII, Fig. 3, respectively), with UO~ ÷ and Pu 4÷. Improvements of 7.1, 6.2 and 6.6 log units, respectively, were observed in going from UO~ ÷ to Pu 4+, indicating that N donors could be used to separate these actinides. This ligand design concept (that of selective complexing of large metal ions) was used by Jarvis et al. (1996) to synthesize a chelating resin. Manufacture involved the functionalization of chloromethylated polystyrene with diethanolamine. The resin exhibited selectivity for Pu(IV) which was similar to that shown by hydroxylamine ligands in solution. This property was confirmed by the resin's ability to suppress hydrolysis up to pH 6.0 (Jarvis et al., 1996). The addition of neutral O donors, either as pendant groups attached to an amine or as a crown ether macrocycle, has proved a useful tool in the progressive design of ligands, which demonstrate enhanced selectivity towards large metal ions (e.g. Pb 2÷, Cd 2÷, Pu 4÷) (Hancock et al., 1989). An important phenomenon in crown ether and calixarene chemistry has been that of selectivity by matching cavity size with metal ion size, as described earlier in this review. Hancock et al. (1989) indicated that the size-selective coordination geometry associated with neutral O donor groups, to a large extent, determined the size selectivity of macrocyclic ligands. Thus, it was found that "large metal ions show increased complex stability when groups containing

neutral O donors are added to an existing ligand, whereas small metal ions tend to show decreases in complex stability." Based on experimental evidence, it was concluded that, although macrocyclic structures may sharpen size selectivity, O donors need not be part of a macrocyclic ring in order to generate selectivity for metal ions. A more important criteria was "that a sufficient number of O donor atoms should be added so that a critical level of steric crowding around the metal ion is achieved" (Hancock et al., 1989) The apparently lower than expected formation constant for Pu(IV) with THEC (1,3,8,11tet rakis(2-hydroxyethyl)- 1,4,8,11-tetraazacyclotetradecane, structure XXXVI, Fig. 3, log [3PuTHEC = 17.39) was explained in terms of both fiveand six-membered rings being formed. Therefore THEC required certain design modifications from a tetra-N-substituted cyclam moiety to a cyclen-based chelator in order to restrict ring formation to five-membered rings and to increase its plutonophile nature. By applying these principles, a superior ligand design (THP-12aneN4) was recommended by Jarvis (1994) as providing less strain energy with larger metal ions (featuring long metal to ligand bonds of about 2.5 A and small L-M-L bond angles approximating to 70°). It has therefore been suggested that THP-12aneN4 (structure XXXVIII, Fig. 3) would be an excellent sequestering agent for Pu(IV), with optimum selectivity occurring in alkaline media (Jackson and Jarvis, 1994). Nevertheless, a possible limitation is that high selectivity factors (e.g. K~v)/KM°+) for larger metal ions do not always accompany a large increase in formation constant value. Stability enhancement has thus been accomplished via pendant amide coordinates wherein an increase in inherent strength factor may be required (Carlton et al., 1994). Based upon interactions with Cd(II) and other large metal ions, 13values for Pu(IV)-TCM-12aneN4 complexation are expected to be high ([3 > 102~) (Jarvis, 1994). The macrocyclic ligand TCM- 12aneN4 (structure XXXIX, Fig. 3) also indicates remarkable selectivity for metal ions having an ionic radius greater than 0.9/~ (Carlton et al., 1994). With respect to structure and coordination, the ~3C N M R spectrum [Cd(L)](CIO4)2 in CH3OH as solvent strongly suggests that this complex closely resembles square-antiprismatic stereochemistry, which is ideal for an eight-coordinate complex. Jarvis (1994) concluded that ligands incorporating these features would be selective for Pu(IV) at a lower pH, compared with those bearing neutral O donors.

Concluding Remarks Feasibility o f a Pu-calixarene complex

The water soluble, p-sulphonate calix[6]arene complex has proven to be a highly efficient

A review of plutonium(IV) selective ligands uranophile (Hall et al., 1994, 1996). Such uranophilic behaviour was characterized by the unusual tendency of the uranyl ion to coordinate to ligands in a hexa-coordinate quasi-planar configuration. Due to the unique size exclusion of the 1:1 complex, observed during selectivity studies, both in solution and on a polymeric backbone (Hall and Williams, 1994; Webb, 1995), it is unlikely that calix[6]arene will exhibit the same "ideal' geometric specificity towards Pu(IV) or Pu(VI). Despite these restrictions, the complexing ability of two substituted calix[4]arenes might be considered as a feasible option. By using esterification and sulphonation to attach hydrophilic and ligating functional groups (e.g. amide, carboxylate and sulphonate groups) to the upper and lower rims of p-tert-butylcalix[4]arene, these molecules could be made to tailor the coordination preferred by Pu (Gutsche, 1989).

197

affinity for Pu(IV) with good selective properties; (ii) the ability to complex Pu(IV) under highly acidic conditions; (iii) published protonation constants (pKa) values; and (iv) it is commercially available and easily regenerated. This chelating resin appears to be a prime candidate for future Pu(IV)-scavenging studies. 3. The substituted N-macrocycles in category 5 (TCM-12aneN4 and THP-12aneN4) are considered to be very encouraging prototypes for new Pu chelating resins of type (iii)*. These ligands show remarkable selectivity for large metal ion and form five-membered chelate rings. TCM-12aneN4, bearing pendant amide coordinates, is expected to be equally selective for Pu(IV) at lower pH, compared with analogous ligands bearing normal O donors. Preliminary results indicate that immobilization of N-macrocycles like cyclen (12aneN4) on chloromethylated polystyrene is possible (Jarvis, 1994).

Superior plutonophiles

The requirements imposed on Pu(IV) extractants are often dictated by the unique specifications of a particular nuclear establishment which produces a range of contaminated waste solutions. Because the disposal of excess organic effluent is an expensive and formidable task, there is a growing emphasis away from the use of solvent extraction. The preferred option, therefore, is to immobilize Pu(IV), using reversed phase extraction or ion-exchange techniques. The future of Pu isolation may be seen to lie with a single stage, two step process in which the element is quantitatively removed from the waste solution using a Pu(IV)-specific ligand immobilized on a solid support. The bound Pu being subsequently released, quantitatively, by some simple manipulation, such as a pH change, from the insolubilized ligand for recycling or long-term storage. This should offer a cost-effective and efficient means of separation and recycling. Based on these premises we conclude that, of the range of potential plutonophiles reviewed, three types merit further investigation. 1. The siderophoric derivative formed by the addition of one hydroxypyridonate moiety to desferrioxamine--DFO-HOPO, which is highly Puspecific, thus demonstrating chelation properties similar to a resin type iii*. This molecule exhibits: (i) a potency for the chelation of Pu(IV); (ii) a relatively high acidity combined with good solubility properties; and (iii) the ability to satisfy the eight-coordinate stereochemistry of Pu(IV). 2. The novel resin type (ii)* actinide-chelating ligand-diphonix resin, which contains diphosphonic acid groups bonded to a sulphonated, styryl-divinyl benzene back bone, demonstrates: (i) a very strong *See ion exchange earlier.

Further research in these areas appears to offer good possibilities for developing new agents which will permit the isolation of Pu from liquid wastes of various types on either a small or large scale. The use of ligands immobilized on resins should permit the isolation of Pu in a form which it can readily be released for recycling or for conversion into chemical forms suitable for long-term storage. Acknowledgements--This work has been carried out with the support of the Procurement Executive of the U.K. Ministry of Defence as part of an extramural contract. The authors also wish to thank Dr N. Jarvis (AEC.AEK), Dr N. Stradling (NRPB) and Miss C. Evans [AWE plc).

References Agarwal R. P. and Mehrotra R. C. (1963) Catechol derivatives of thorium. J. Inorg. Nucl. Chem. 24, 821. Alexandratos S. D., Wilson D. L., Kaiser P. T. and McDowell W. J. (1987) Dual mechanism bifunctional polymers: phosphinic acid ion-exchange/redox resins and the kinetics of metal-ion complexation. React. Polvm. 5, 23. Baracus A., Baroncelli F., Biondi G. B. and Grossi G. (1966) The complexingpower of hydroxamic acids and its effect on behaviour of organic extractants in the reprocessing of irradiated fuels--II. J. lnorg. Nucl. Chem. 28, 2961. Blight D. G. and Kepert D. L. (1972) The stereochemistry of eight coordination. The effect of bidentate ligands. lnorg. Chem. 11, 1556. Bossier P., Hofte M. and Verstraete W. (1988) Ecological significance of siderophores in soil. Adv. Microbial Ecol. 10, 385. Brainard J. R., Strietelmeier B. A.. Smith P. H., Langston-Unkefer P. J., Barr M. E. and Ryan R. R. (1992) Actinide binding and solubilization by microbial siderophores. Radiochim. Acta 58]59, 35Z Brown D., Holah D. G. and Rickard C. E. F. (1970) Structure of thorium(IV) tetrakis-(N,N-diethyldithiocarbamate). J. Chem. Soc. A, 423. Calmon C. and Simon G.P. (1979) The ion-exchang¢rs. In Ion Exchange for pollution control (Calmon C. and Gold H., Eds), Vol. 1, Chap. 2. CRC Press, Boca Raton, Fla.

198

N. C. O'Boyle et al.

Carlton L., Hancock R. D., Maumela H. and Wainwright K. P. (1994) Stability enhancement of heavy-metalmacrocycle complexes via pendant amide coordination. J. Chem. Soc. Chem. Commun., 1007. Chiarizia R., Horwitz E. P., Gatrone R. C., Alexandratos S. D., Trochimczuk A. Q. and Crick D. W. (1993) Uptake of metal ions by a new chelating ion-exchange resin. Part 2: acid dependencies of transition and post-transition metal ions. Sob,. Extract. Ion Exch. !1, 967. Chiarizia R., Horwitz E. P. and Alexandratos S. D. (1994) Uptake of metal ions by a new chelating ion-exchange resin. Part 4: kinetics. Soh,. Extract. Ion Exch. 12, 211. Chiarizia R. and Horwitz E. P. (1994) Uptake of metal ions by a new chelating ion-exchange resin. Part 6: calculations on the effect of complexing anions as actinides. Soh,. Extr. Ion Exch. 12, 831. Chmutova M. K. and Zolotov Yu. A. (1964) Investigation of the extraction and complex formation of plutonium(IV) with N-benzophenylhydroxylamine. Sot,. Radiochem. (Engl. Transl.) 6, 625. Coutts R. T. (1967) Hydroxamic acids I. Acyclic hydroxamic acids. Can. J. Pharmac. Sci. 2, 1. Coutts R. T. (1967) Hydroxamic acids II. Cyclic hydroxamic acids. Can. J. Pharmac. Sci. 2, 27. Duffield J. R. and Williams D. R. (1986) The environmental chemistry of radioactive waste disposal. Chem. Soc. Rel,. 15, 291. Durbin P. W., Kullgren B., Xu J. and Raymond K. N. (1994) In l'it,o chelation of Am(Ill), Pu(IV), Np(V) and U(VI) in mice by Tren-(Me-3,2-HOPO). Radiat. Protect. Dosimetrl, 53, 305. Exner O. (1968) Physicochemical investigations on hydroxamic acids. Baggesgaard lecture. Dan. Tidsskr. Farm. 42, 145. Foreman H. (1960) The application of chelating agents for hastening excretion of radioelements. In Metal Binding in Medicine (Seven M.J. and Johnson L.A., Eds), p. 160. Lippincott, Montreal. Foreman H. and Finnegan C. (1957) Effect of ethylenediaminetetraacetic acid on deposition and excretion of certain rare earth elements. J. Biol. Chem. 226, 745. Foreman H. and Hamilton J. G. (1951) The use of chelating agents for accelerating excretion of radionuclides. AECD 3247, 1. Fritsch P., Herbreteau D., Moutairou K., Lantenois G., Richard-Le Naour H., Grillon G., Hoffschir D., Poncy J. L., Laurent A. and Masse R. (1994) Comparative toxicity of 3,4,3-LIHOPO and DTPA in baboons: preliminary results. Radiat. Prot. Dosimet. 53, 1. Gopalan A. S., Huber V. J., Zincircioglu O. and Smith P. H. (1992) Novel tetrahydroxamate chelators for actinide complexation: synthesis and binding studies. J. Chem. Soc. Chem. Commun. 17, 1266. Gray S. A., Stradling G. N., Pearce M. J., Wilson 1., Moody J. C., Burgada R., Durbin P. W. and Raymond F. N. (1994) Removal of plutonium and americium from the rat using 3,4,3-LIHOPO and DTPA after simulated wound contamination. Effect of delayed administration and mass of plutonium. Radiat. Protect. Dosimet. 53, 315. Gutsche C.D. (1989) Calixarenes (Stoddart, J.F., Ed.). The Royal Society of Chemistry, Cambridge. Hall I. and Williams D.R. (1994) Unpublished data. University of Wales, Cardiff. Hall I., Nicholson G. P., Piper T. J., Williams D. R. and Williams G. (1994) The scavenging or uranyl from phosphate-containing media. Appl. Radiat. Isot. 45, 1065. Hall I., Nicholson G. P., Piper T. J., Williams G. and Williams D. R. (1996) Coordination chemistry and speciation analysis of a uranyl-calix[6]arene complex in phosphate-enriched media. Radiochim. Acta 69, 225. Hancock R. D., Bhavan R., Wade P. W., Boeyens J. C. A. and Dobson S. M. (1989) Ligand design for complexation in aqueous solution. 1. Neutral oxygen donor beating

groups as a means of controlling size-based selectivity for metal ions. Inorg. Chem. 28, 187. Hancock R. D. and Martell A. E. (1989) Ligand design for selective complexation of metal ions in aqueous solution. Chem. Re~'. 89, 1875. Horwitz E. P., Chiarizia R., Diamond H., Gatrone R. D., Alexandratos S. D., Trochimczuk A. Q. and Crick D. W. (1993) Uptake of metal ions by a new chelating ion-exchange resin. Part 1: acid dependencies of actinide ions. So&. Extr. Ion Exch. 11(5), 943. Horwitz E. P., Chiarizia R. and Alexandratos S. D. (1994) Uptake of metal ions by a new chelating ion-exchange resin. Part 5: the effect of solution matrix on actinides. So&. Extr. Ion. Exch. 11(4), 831. Jackson G.F. and Jarvis N.V. (1995) Speciation as a tool for chelation treatment strategies. In Handbook of Metal-Ligand Interactions in Biological Fluids (Berthon F., Ed.), Vol. 2, Part 5, Chap. 4, p. 206. Dekker, New York. Jarvis N. V. and Hancock R. D. (1994) The affinity of plutonium(IV) for nitrogen donor ligands. Radiochim. Acta 64, 15. Jarvis N (1995) Private Communication. Jarvis N. V., Wagener J. M. and Hancock R. D. (1995) Evaluation of a diethanolamine chelating resin using two-phase potentiometry. Soh,. Extr. Ion E.,:ch. 13(3), 591-611. Katz J. J., Seaborg G. T. and Morss L. R. (1986) The Chemistry of the Actinides (2nd Edn), Vol. 2. Chapman and Hall, London. Kawin B. and Copp D. H. (1953) Effect of 2, 3-dimercaptopropanol on distribution and excretion of plutonium. Proc. Soc. Exptl Biol. Med. 84, 576. Kepert D. L. (1978) Aspects of stereochemistry of eight-coordination. Prog. Inorg. Chem. 24, 179. Kim J. A. and Kanellakopulos B. (1989) Solubility products of plutonium(IV) oxide and hydroxide. Radiochim. Acta 48, 145. Koshti N., Huber V., Smith P. and Gopalan A. S. (1994) Design and synthesis of actinide specific chelators: synthesis of new cyclam tetrahydroxamate (CYTROX) and cyclam tetraacetonylacetone (CYTAC) chelators. Tetrahedron 50(9), 2657. Korkish J. (1989) Handbook of Ion-Exchange Resins: Their Application to Inorganic Analytical Chemistry. CRC Press, Boca Raton, Fla. Kroll H (1956) Development of chelating agents potentially more effective than ethylenediaminetetraacetic acid in radioelement removal. U. S. Atomic Energy Communication (Argonne National Laboratory Report), ANL55841956150-151. Krot N. N., Ermulaev N. P. and Gel'man A. D. (1962) Behaviour of ethylenediaminetetraacetic acid in acid solutions and its reaction with U(IV). Russ. J. Inorg. Chem. 7, 1062. Livet J., Cordier P.Y. and Condamines N (1993) 23bmes Journbes des Actinides. Schwarzwald. Liyanage J. A., Taylor D. M. and Williams D. R. (1995) Spectrophotometric analysis of uranyl as its 4-sulfonic calix[6]arene complex. Analyt. Proc. incl. Analyt. Commun. 32, 217. Lloyd R. D., Bruenger F. W , May C. W., Atherton D. R., Jones C. W., Taylor G. N. and Stevens W. (1984) Removal of Pu and Am from beagles and mice by 3,4,3-LICAM(C) or 3,4,3-LICAM(S). Radiat. Res. 99, 106. Manchanda V. K. and Mohapatra P. K. (1990) Extraction of plutonium(IV) with ionisable macrocyclic ligands. Radiochim. Acta 50, 209. Marsh S.F. and Simi O.R. (1981) Applications of DHDECMP extraction chromatography to nuclear analytical chemistry. In Analytical Chemistry in Nuclear Technology Proceedings of the Twenty-fifth Conference on

A review of plutonium(IV) selective ligands Analytical Chemisto' in Energy Technology (Lyon W.S., Ed.), Oak Ridge National Laboratory, Tenn. Mathis F. (1953). ftvdroxamic acids. Bull. Soc. Chim. Fr.D., 9. Mathis F., Mathis-Neol R., Chauveau A. and Munoz A. (1961) Organic hydroxylamine derivatives. Ann. Fae. Sei. Unw. Toulouse Sci. Math. Sci. Phys. 25, 125. Mathur J. N., Murali M. S. and Natarajan P. R. (1992) Extraction of actinides and fission products by octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide from nitric acid media. Talanta 39, 493. Monzyk B. and Crumbliss A. L. (1979) Mechanism of ligand substitution on high-spin iron(III) by hydroxamic acid chelators. Thermodynamic and kinetic studies on the formation and dissociation of a series of monohydroxamate iron(Ill) complexes. J. Am. Chem. Soc 101(21), 6203. Monzyk B. and Crumbliss A. L. (1980) Acid dissociation constants (K,) and their temperature dependencies (AK,, AS.,) for a series of carbon- and nitrogen-substituted hydroxamic acids in aqueous solution. J. Org. Chem. 45, 4670. Musikas C. (1984) Actinide Lanthanide Separations (Choppin G.R., Navratil J.D. and Schultz W.W., Eds). World Scientific, New York. Nash K. L., Rickert P. G., Muntean J. V. and Alexandratos S. D. (1994) Uptake of metal ions by a new chelating ion-exchange resin. Part 3: protonation constants via potentiometric titration and solid state 3~p NMR spectroscopy. Soh,. Extr. Ion Exch. 12, 193. Nebel D. (1966) Spectrophotometric studies of equilibrium of Pu(IV) citrate in aqueous solution. Z. Phys. Chem. (Leipzig) 232, 161. Nigond L., Condamines N., Cordier P. Y., Liver J., Madic C., Cuillerdier C. and Musicas C. (1995) Recent advances in the treatment of nuclear wastes by the use of diamide and picolamide extractants. Separ. Sci. Technol. 30, 7-9, 2075. Neu M. P., Hoffman D. C., Roberts K. E., Neitsche H. and Silva R. J. (1994) Comparison of chemical extractions and laser photoacoustic spectroscopy for the determination of plutonium species in near-neutral carbonate solutions. Radiochim. Acta 66-67, 251. O'Boyle N. and Williams D.R. (1995) Ligands for plutonium extraction. Unpublished data, University of Wales, Cardiff. Painter E., Russell E., Prosser C.L., Swift M.N., Kisieleski W. and Sacher G. (1946) Chemical physiology of dogs injected with plutonium. Chicago University Metallurgical Laboratory Report, CH-3858 (AECD-2042). Paquet F., Poncy J. L., Rateau G., Burgada R., Bailly T., Leroux Y., Raymond K. N., Durbin P. W. and Masse R. (1994) Reduction of the retention of :3Spu in rats treated by 3,4,3-LIHOPO. Radiat. Protect. Dosimet 53, 323. Pearson R. G. (1990) Hard and soft acids and bases--the evolution of a chemical concept. Coordin. Chem. Rev. 100, 403. Phillips S. L., lgbene A., Fair J. A., Ozbek H. and Tavana M. (1982) A technical data book for geothermal energy utilization, LBL-14313. Proc. Symp. Tkermophys. Prop. g, 345. Piskunov E. M. and Rykov A. G. (1972) Complexing with diethylenetriaminepentaacetic acid V. Plutonium (IV) Radiokhim. 14, 332. Poncy J. L., Rateau G., Burgada R., Bailly T., Leroux Y., Raymond K. N., Durbin P. W. and Masse R. (1993) Efficacy of 3,4,3-LIHOPO for reducing the retention of -'38Pu in rat after inhalation of the tributyl phosphate complex. Int. J. Radiat. Biol. 64, 431. Rai D. (1984) Solubility product of Pu(IV) hydrous oxide and equilibrium constants of Pu(IV)/Pu(IV) and Pu(IV)/ Pu(VI) couples. Radiochim. Acta 35, 97. Raymond K.N. and Smith W.L. (1981) Actinide-specific sequestering agents and decontamination applications.

199

Structure and Bonding (Goodenough, J.B., Ed.), Vol. 43, p. 159. Springer, Berlin. Rizkalla E. N. (1983) Metal chelates of phosphonate-containing ligands. Rev. Inorg. Chem. 5, 223. Rogers R. D., Bond A. H., Hipple W. G., Rollins A. N. and Hendy R. F. (1991) Synthesis and structural elucidation of novel uranyl-crown ether compounds isolated from nitric, hydrochloric, sulfuric and acetic acids, lnorg. Chem. 30, 2671. Roth P. C. and Raymond K. N. (1995) Amide func!ional group contribution to the stability of gadolinium(Ill) complexes: DTPA derivatives. Inorg. Chem. 34, 1408. Rydberg J. (1956) Extraction of metal complexes XXV. Complex formation of Pu(IV) with acetylacetone, lnorg. Chem. 34, 109. Sandler S.R. and Karo W. (1972) Organic Functional Group Preparation, Vol. 8, p. 406. Academic Press, New York. Schubert J. and Wallace H. Jr (1950) The effect of zirconium and sodium citrate on the distribution and excretion of simultaneously injected thorium and radio-strontium. J. Biol. Chem. 183, 157. Shukla J. P. and Lohithakshan K. V. (1989) Selective liquid-liquid extraction of uranium(VI) and plutonium(IV) from aqueous nitric acid media by crown ethers. Chem. Scripta 29, 341. Shukla J. P., Singh R. K. and Kumar A. (1991) Extraction of uranium(Vl) and plutonium(IV) into toluene by crown ethers from nitric acid solution. Radiochim. Acta 54, 73. Shukla J. P., Kumar A. and Singh R. K. (1993) Liquid-liquid extraction of plutonium(IV) by dicyclohexano-18-crown-6 from aqueous-organic solutions. Radiochim. Acta 60, 103. Smith, P.A.S. (1966) The Chemist O' of Open-Chain Organic Nitrogen Compoundg, Vol. 2, p. 68. Benjamin, New York. Sofen S. R., Cooper S. R. and Raymond K. N. (1979) Crystal and molecular structures of tetrakis (catecho[ato) hafnate(IV) and cerate(IV). Further evidence for a ligand field effect in the structure of tetrakis (catecholato) uranate(IV). Inorg. Chem. 18, 1611. Sofen S. R., Abu-Dari K., Freyberg D. P. and Raymond K. N. (1978) Specific sequestering agents for the actinides 2. A ligand field effect in the crystal and molecular structures of tetrakis (catecholato) uranate(IV) and thorate(IV). J. Am. Chem. Soe. 100, 7882. Steffen W. L. and Fay R. C. (1978) Reinvestigation of the coordination geometry of eight-coordinate metal tetratis (acetyl acetonates). Inorg. Chem. 17, 779. Stradling G. N., Gray S. A., Pearce M. J., Wilson I., Moody J. C., Burgada R., Durbin P. W. and Raymond K. N. (1995) Decorporation of thorium-228 from the rat by 3,4,3-LIHOPO and DTPA after simulated wound contamination. Hum. Exptl Toxicol. 14, 165. Stradling G.N., Gray S.A., Pearce M.J., Wilson I., Moody J.C., Hodgson A. and Raymond K.N. (1995b) EffiCiency of TREN-(Me-3,2 -HOPO), 5-LI-(Me-3,2-HOPO and DTPA for removing plutonium and americium from the rat after inhalation and wound contamination as nitrates: comparison with 3,4,3-LI(I,2-HOPO). Memorandum, no. NRPB-M534, Chilton. Stradling G. N. (1994) Recent progress in decorporation of plutonium, americium and thorium. Radiat. Prot. Dosimet. 53, 297. Stradling G. N., Gray S. A., Moody J. C., Pearce M. J., Wilson I., Burgada R., Bailly T., Leroux Y., Raymond K. N. and Durbin P. W. (1993) Comparative efficacies of 3,4,3-LIHOPO and DTPA for enhancing the excretion of plutonium and americium from the rat after simulated wound contamination as nitrates. Int. J. Radiat. Biol. 64, 133. Stradling G. N., Gray S. A., Ellender M., Moody J. C., Hodgson A., Pearce M., Wilson I., Burgada R., Bailly T., Leroux Y. G. P., El Manouni D., Raymond K. N. and Durbin P. W. (1992) The efficacies of 3,4,3-LIHOPO and

200

N. C. O'Boyle et al.

DTPA for enhancing the excretion of plutonium and americium from the rat, comparison with other siderophore analogues. Int. J. Radiat. Biol. 62, 487. Taylor D. M. (1995) Environmental plutonium in humans. Appl. Radiat. lsot. 46, 1245. Taylor D. M. (1989) The biodistribution and toxicity of plutonium, americium and neptunium. Sci. Total Era,iron. 83, 217. Turowski P. N., Rodgers S. J., Scarrow R. C. and Raymond K. N. (1988) Ferric ion sequestering agents 18. Two dihydroxamic acid derivatives of EDTA and DTPA. lnorg. Chem. 27, 474. Vaughan J., Bleany B. and Taylor D. M. (1973) Distribution, excretion and effects of plutonium as a bone-seeker. Handbk Exptl Pharmacol. 30, 349. Volf V., Taylor D. M., Brandau W. and Schlenker P. (1986) Rapid communication: the effect of pure LICAM(C) on the retention of plutonium-238 in mice and rats. Int. J. Radiat. Biol. 50, 205. Volf V., Burgada R., Raymond K. N. and Durbin P. W.

(1993) Early chelation therapy for injected Pu-238 and Am-241 in the rat: comparison of 3,4,3-LIHOPO, DFO-HOPO, DTPA-DX. D T P A DFOA. Int. J. Radiat. Biol. 63, 785. Webb L.M. (1995) A sequestering resin for uranyl scavenging. MPhil Thesis, University of Wales, Cardiff. Weitl F. L., Raymond K. N., Smith W. L. and Howard T. R. (1978) Specific sequestering agents for the actinides 1. N,N',N",N'"-tetra(2,3-dihydroxybenzoyl) tetraazacyclo-tetra-and hexadecanes. J. Am. Chem. Soc. 100, 1170. Weitl F. L. and Raymond K. N. (1980) Specific sequestering agents for the actinides 3. Polycatecholate ligands derived from 2,3-dihydroxy-5-sulfobenzoyl conjugates of diaza-and tetraazaalkanes. J. Am. Chem. Soc. 102, 2289. White D. L., Durbin P. W., Jeung N. and Raymond K. N. (1988) Specific sequestering agents for the actinides 16. Synthesis and initial biological testing of polydentate oxohydroxypyridinecarboxylate ligands. J. Med. Chem. 31, 11.